DEBRIS FLOWS VS. SHEETFLOODS: HOW FIRE, VEGETATION AND CLIMATE CONTROL EROSIONAL RESPONSE IN SMALL STEEP BASINS

Changes in climate influence vegetation distributions and the frequency and severity of wildfire; fire and vegetation changes are subsequently reflected in the erosional response of the landscape. This study investigates how the type of erosional response (e.g. large debris flows vs. small sheetfloods) reflects climate-driven spatial and temporal changes in vegetation and fire activity in the Middle Fork Salmon River (MFSR) Idaho. The MFSR watershed contains subalpine pine and spruce forests (~3,000 -1,700 m), montane Douglas-fir and ponderosa pine-dominated forests (2650 - 1130 m) and sagebrush steppe (~ 1,800 - 900 m). Fire reconstructions from ¹⁴C dating of 45 charcoal fragments found in incised alluvial fans indicate the warm, dry mid-Holocene (8500-5500 cal yr BP) lacks fire-related debris flow deposits; however, synchronous high and low elevation fire-related sheetfloods occurred ~5800 and 6500 cal yr BP. Deposits compose ~4% of the total dated deposit thickness suggesting lower sediment transport during drier/warmer climate. In contrast, cool, wet, and more variable climate conditions between 14000-8500 cal yr BP and during the late Holocene (< 4500 cal yr BP) are associated with large fire-related debris flows. Deposition during these two periods compose ~22% and ~74% of total dated fan thickness suggesting increased sediment transport from small steep tributary basins to large trunk streams during cooler/wetter climate conditions. Small sheetflooding events are more common and compose greater fan thickness (~25%) in lower elevation basins, compared to <1% in forested basins. We hypothesize that the presence of sheetflood deposits from both high and low elevation basins and the lack of debris flow deposits between 8500-5500 cal yr BP reflects climate-driven changes in both vegetation and fire behavior. During warm dry conditions, vegetation density decreases, fire severity decreases, ecosystems adapt to a warmer drier climate with more fire resistant species (e.g. Douglas fir), and sediment transport is decreased. Conversely, during the cooler/moister early and late Holocene, inferred increased vegetation density fueled greater fire severity as demonstrated by large debris flows, and resulted in accelerated erosion.